Spring Drought/Flood Outlook
for the Lower Rio Grande Valley


April Update: Dry, Increasingly Hot Weather Expected to Continue

Existing Conditions
Rainfall: Rainfall totals have been below normal during the winter and early spring months (December - March) over Deep South Texas and the Lower Rio Grande Valley. During this time period, rainfall totals have averaged 3 inches or less over the region. Rainfall over the region has been below normal since August 2007.

Soil Moisture: The combination of below normal rainfall…above normal temperatures and abundant sunshine have maintained below normal soil moisture values over Deep South Texas and the Lower Rio Grande Valley through March. Daily evaporation rates during the past months have ranged from a quarter of an inch to near half an inch. These current evaporation rates will likely persist or steadily increase over the next several months.

Reservoir Conditions: Water storage levels at both the Falcon Reservoir and Lake Amistad are expected to continue decreasing during the next several weeks in response to an increase in water releases and demand. These releases are expected as peak irrigation season continues over the Lower Rio Grande Valley.

Drought Conditions: Moderate to severe drought conditions continue over much of Deep South Texas. Zapata, Starr, Jim Hogg, western Brooks, and western Hidalgo counties are all currently experiencing severe drought conditions. The rest of Deep South Texas remains under moderate drought conditions. These moderate to severe drought conditions are expected to persist or worsen across the remainder of Deep South Texas through the rest of this spring.

Meteorological Outlook
Guidance from the Climate Prediction Center (CPC) indicates that the current moderate La Niña pattern in the tropical Pacific Ocean will persist throughout April 2008. This will likely maintain a very warm and dry weather pattern over Deep South Texas and the Lower Rio Grande Valley. This pattern will continue a fairly steady progression of storm systems moving across the United States, generally from west to east through this month. This progression of storm systems will usher a couple of weak to moderate cold fronts across the area over the next few weeks. However, overall atmospheric moisture values with these systems will be fairly limited for Deep South Texas and the Lower Rio Grande Valley. Accordingly, only isolated to scattered shower and thunderstorm activity is expected across the area from April 9th through April 12th. Any rainfall that occurs during this period will not be enough to make a notable dent to the current dry conditions. Significant and widespread soaking rainfall is not expected throughout the rest of April 2008.

Long Term Outlook
Longer range guidance from CPC indicates that the moderate La Niña pattern will likely continue throughout the rest of spring 2008. This persistent pattern will in turn maintain above normal temperatures and below normal rainfall for the region through May. As a result, drought conditions are expected to persist and even intensify across Deep South Texas and the Lower Rio Grande Valley. However, CPC expects that the current La Niña pattern could weaken some as we move into the summer months. This weakening La Niña pattern may allow for somewhat increased rain chances during the early summer, which may provide some improvement of the current drought conditions affecting the region. However, near normal to above normal temperatures may continue into the summer months for Deep South Texas and the Lower Rio Grande Valley.

Spring Flood Outlook
With aggregate rainfall forecasts to remain below average for the next several months, the potential for spring flooding is considered low. That said, one must remain alert to localized slow moving or stationary thunderstorms as we move toward summer. Such storms can produce very heavy amounts of rainfall in a short period of time, causing flash flooding of poor drainage areas, arroyos, and other prone locations.

Below are graphical data showing, from left: The April 1st Texas Drought Monitor; The latest April through June forecast temperature departures from average, and the latest April through June forecast precipitation departures from average. The Rio Grande Valley is inside the small box. Click each for a larger image.

Texas Drought Monitor, April 1, 2008 (click to enlarge) Climate Prediction Center probability of departure from normal, temperature, April through June, 2008 (click to enlarge) Climate Prediction Center probability of departure from normal, rainfall, April through June, 2008 (click to enlarge)
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